全文获取类型
收费全文 | 28947篇 |
免费 | 110篇 |
国内免费 | 275篇 |
专业分类
化学 | 10546篇 |
晶体学 | 267篇 |
力学 | 1378篇 |
综合类 | 12篇 |
数学 | 9027篇 |
物理学 | 8102篇 |
出版年
2021年 | 37篇 |
2020年 | 40篇 |
2019年 | 31篇 |
2018年 | 1201篇 |
2017年 | 1455篇 |
2016年 | 716篇 |
2015年 | 564篇 |
2014年 | 479篇 |
2013年 | 650篇 |
2012年 | 3136篇 |
2011年 | 2311篇 |
2010年 | 1793篇 |
2009年 | 1531篇 |
2008年 | 544篇 |
2007年 | 604篇 |
2006年 | 597篇 |
2005年 | 4431篇 |
2004年 | 3911篇 |
2003年 | 2295篇 |
2002年 | 485篇 |
2001年 | 287篇 |
2000年 | 94篇 |
1999年 | 170篇 |
1998年 | 110篇 |
1997年 | 69篇 |
1996年 | 45篇 |
1995年 | 41篇 |
1994年 | 44篇 |
1992年 | 174篇 |
1991年 | 154篇 |
1990年 | 135篇 |
1989年 | 109篇 |
1988年 | 99篇 |
1987年 | 60篇 |
1986年 | 44篇 |
1985年 | 43篇 |
1984年 | 33篇 |
1983年 | 30篇 |
1979年 | 30篇 |
1978年 | 27篇 |
1976年 | 84篇 |
1975年 | 40篇 |
1974年 | 42篇 |
1973年 | 51篇 |
1972年 | 41篇 |
1970年 | 35篇 |
1969年 | 37篇 |
1968年 | 35篇 |
1967年 | 30篇 |
1966年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
A study was made on the inhibition by some 3,3′- and 4,4′-disubstituted azobenzenes of the polymerization of vinylacetate initiated by azoisobutyronitrile at 50°. The inhibitory effects of these substances can be attributed to their ability to engage in radical addition giving a less reactive hydrazyl type radical. The mechanism of the inhibition has been established by ESR and kinetic (stoichiometric) measurements. The value of k5/k2 (characteristic of the reactivity of an inhibitor) was determined for 9 substituents. The radical reactivity of the aromatic azo group was decreased by electron donor substituents and increased by electron acceptors. The substituent effect can be well interpreted by the Hammett equation; the value of the reaction constant was ? = +0.53. 相似文献
62.
A solid-phase reagent based on 1-(4-adamantyl-2-thiasolylazo)-2-naphthol adsorbed onto silica gel was prepared for Co(II) recovery and preconcentration prior to its sorption-spectroscopic detection. The immobilized reagent was applied to the determination of free cobalt ions in natural water. The solid-phase reagent and chemiluminescent method coupled with membrane filtration, gel-permeation and ion-exchange chromatography were applied to the study of the speciation of iron and cobalt in water from the Dnieper reservoirs and lakes of Kyiv City; their predominant forms were complexes of Fe(III) and Co(II) with dissolved organic matter and fulvic acids play a main role in their complexation. 相似文献
63.
The 2-methyloxazol-5-on-2-yl radical (3) and its deuterium labeled analogs were generated in the gas-phase by femtosecond electron-transfer and studied by neutralization-reionization mass spectrometry and quantum chemical calculations. Radical 3 undergoes fast dissociation by ring opening and elimination of CO and CH(3)CO. Loss of hydrogen is less abundant and involves hydrogen atoms from both the ring and side-chain positions. The experimental results are corroborated by the analysis of the potential energy surface of the ground electronic state in 3 using density functional, perturbational, and coupled-cluster theories up to CCSD(T) and extrapolated to the 6-311 ++ G(3df,2p) basis set. RRKM calculations of radical dissociations gave branching ratios for loss of CO and H that were k(CO)/k(H) > 10 over an 80-300 kJ mol(-1) range of internal energies. The driving force for the dissociations of 3 is provided by large Franck-Condon effects on vertical neutralization and possibly from involvement of excited electronic states. Calculations also provided the adiabatic ionization energy of 3, IE(adiab) = 5.48 eV and vertical recombination energy of cation 3(+), RE(vert) = 4.70 eV. The present results strongly indicate that oxazolone structures can explain fragmentations of b-type peptide ions upon electron capture, contrary to previous speculations. 相似文献
64.
1-Aryl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolines were prepared according to a one-pot procedure involving the reaction of 2-(3,4-dimethoxyphenyl)-ethylamine with aromatic aldehydes in TFA at reflux. The tetrahydroisoquinolines were treated with H2O2-WO42− in methanol at room temperature to give the corresponding 3,4-dihydroisoquinoline-2-oxides. Treatment of these cyclic nitrones with DMAD in toluene at room temperature gave the corresponding isoxazolo[3,2-a]isoquinolines. These compounds were heated in toluene at reflux to give the corresponding ylides in high yields (Method A). The effect of the substituents on the rate of the rearrangement of such compounds prompted us to discuss a new mechanism involving consecutive C-C bond heterolysis and 1,3-sigmatropic shift. A one-pot reaction involving the treatment of the nitrones with equimolar amounts of DMAD in refluxing toluene also gave the ylides (Method B). The structures of the prepared compounds were elucidated by spectral means and elemental analyses. 相似文献
65.
Aspects of the use of lipophilic gels in manual sample preparation procedures are reviewed. Neutral gels with a controlled hydrophobicity are used for sorbent extraction of non-polar and medium polarity compounds from biological fluids. Acidic amphiphilic compounds can be extracted as ion-pairs with decyltrimethylammonium ions. Solvent or detergent extracts of tissues or faeces can be mixed with hydrophobic gels for transfer of analytes from a solvent to a gel phase, permitting subsequent sample preparation in gel bed systems. Hydrophobic gels, alkyl-bonded silica and polystyrene matrices can be used in series for extraction of compounds with a wide range of polarities. Group fractionations are performed on neutral and ion-exchanging lipophilic gels to yield fractions of neutral, basic and acidic metabolites within selected polarity ranges. Selective isolation of phenolic acids on a strong anion exchanger, of ethynylic steroids on a strong cation exchanger in silver form and of oximes of ketonic steroids on a strong cation exchanger in hydrogen form is possible. A computerized system for automatic sample preparation is also described. It consists of an extraction bed, a cation-exchange column and an anion-exchange column. The pumps and switching valves are arranged so that the columns can operate in series or parallel for isolation of neutral, basic and acidic metabolites of amphiphilic compounds and for regeneration of the column beds. Fractions can be collected, or the effluent from the column beds can be diluted with water to permit sorption on a solid phase. The applicability of the automated method to the analysis of bile acids and metabolites of mono(2-ethylhexyl) phthalate is demonstrated. 相似文献
66.
During recent years high-performance liquid chromatography has become an excellent tool for the determination of antibiotics in biological fluids. Compared with biological assays, the major benefits of this method are specificity and rapidity. In particular, the determination of biologically inactive metabolites emphasizes that this technique plays an outstanding role for the analysis of antibiotics. This paper describes how the method can be used in the analysis of several antibiotics and demonstrates the efficacy of this method for clinical microbiology. Methods for the determination in biological fluids of acylaminopenicillins (azlocillin, mezlocillin, piperacillin and aspoxicillin), quinolones (ciprofloxacine, norfloxacine and ofloxacine), a penem (imipenem) and a cephalosporin (cefixime) are summarized. Furthermore, their application to in vitro studies and their trial in clinical studies are described. 相似文献
67.
A. Laurinavi?ius K. Rep?as A. -R. Va?kevi?ius 《International Journal of Infrared and Millimeter Waves》1995,16(7):1217-1222
Power-voltage and frequency characteristics of power heads for 90–120 GHz TE10-mode millimeter waves are presented. Operation of the power heads is based on thermo-e.m.f. of hot charge carriers in silicon. The head has been tested both in the pulsed and continuous generation regime. The experiments were carried out in the power range of 2·10–3–4 W. In the pulse regime, the shortest pulse duration was 1s. 相似文献
68.
J. B?chler J. Bartke H. Bia?kowska R. Bock D. Brinkmann R. Brockmann P. Buncic S. I. Chase I. Derado V. Eckardt et al. 《Zeitschrift fur Physik C Particles and Fields》1994,61(4):551-561
The intermittency phenomenon is investigated in proton-gold, oxygen-gold, sulphur-gold and sulphur-sulphur collisions at 200 GeV per nucleon. The data were taken with the NA 35 streamer chamber detector at the CERN SPS. The data samples are carefully corrected for double counting of tracks and for contamination from photon conversions, particle decays and secondary interactions. The analysis is carried out in terms of factorial moments, using a new definition, and of correlation integrals. Both methods show the presence of nonstatistical fluctuations (intermittency). The main conclusions are: The observation of the effect for pairs of hadrons with negative charges and its near absence for pairs of opposite charges is consistent with the assumption that Bose-Einstein correlations yield the dominant contribution. The intermittency effect is not proportional to (dn/dy)–1 which would be expected in superposition models. The measured relation between the second and third factorial moment for negative hadrons indicates that genuine 3-particle correlations are small. The predictions of a Monte Carlo simulation which is based on the Lund Fritiof model with the inclusion of Bose-Einstein correlations agree with the experimental results.Deceased 相似文献
69.
Previous animal experiments suggested that the Magos cold vapor atomic absorption spectroscopic (CVAAS) method might overestimate the concentrations of inorganic mercury (I-Hg) in the presence of methylmercury (MeHg). In the present study it is shown that this error is due to a fast degradation of MeHg during the formation of the analytical signal. For brain samples, about 5% of the total amount of MeHg in the reaction vessel is degraded to I-Hg. Speciation of Hg in aqueous solution of MeHg chloride, after purification with ionexchange chromatography using the Magos method, showed that about 9% was I-Hg. Analysis by NMR of MeHg chloride and MeHg hydroxide showed that less than 1% was in the form of I-Hg. The absolute magnitude of the error in the CVAAS method is dependent on the amounts of SnCl2 and MeHg in the reaction vessel; however, the ratio of I-Hg to total (T-Hg) is shown to be independent of the amount of MeHg (25.5–255 ng as Hg) in the reaction vessel. A procedure for corrections is proposed, based on the results from these studies and empirical data from speciation analyses of brain tissue from MeHg-exposed rats and rabbits. 相似文献
70.
Novel shape descriptors for molecular graphs. 总被引:2,自引:0,他引:2
M Randi? 《Journal of chemical information and computer sciences》2001,41(3):607-613
We report on novel graph theoretical indices which are sensitive to the shapes of molecular graphs. In contrast to the Kier's kappa shape indices which were based on a comparison of a molecular graph with graphs representing the extreme shapes, the linear graph and the "star" graph, the new shape indices are obtained by considering for all atoms the number of paths and the number of walks within a graph and then making the quotients of the number of paths and the number of walks the same length. The new shape indices show much higher discrimination among isomers when compared to the kappa shape indices. We report the new shape indices for smaller alkanes and several cyclic structures and illustrate their use in structure-property correlations. The new indices offer regressions of high quality for diverse physicochemical properties of octanes. They also have lead to a novel classification of physicochemical properties of alkanes. 相似文献